Detection, identification, and analysis of analytes using absorption spectroscopy are known at a macroscopic scale. Such processes require light to interact with an examined analyte for a period of time easily achieved at macroscopic levels. Generally, a setup comprises a fixed length, single pass optical path through an analyte, in which some sort of light source inputs light through the optical path, and some sort of detector detects the light at the end of the optical path.
Miniaturizing optofluidic devices to the microscopic scale potentially enables greater portability, lower analyte consumption for testing, and greater potential for massively parallel measurements. Parallel measurements are identical or substantially identical measurement methods performed with multiple replications of the same apparatus. These measurements can be performed simultaneously, in parallel, and with small devices, they can be designed to perform them in massive quantities. Therefore, incorporating advanced fluid handling techniques at the micron scale with highly sensitive photonic devices has the potential to provide compact, effective sensors for lab-on-a-chip tools, which can also be used to perform massively parallel measurements.
At least partly for these reasons, optofluidic techniques in which microfluidics are integrated with photonic components are gaining more use in biosensing and chemical analysis applications. Many optofluidic transduction methods for sensing and analysis have been demonstrated, including refraction, absorbance, fluorescence, surface-plasmon-resonance, and interferometric measurements. Absorbance-based optofluidic techniques are particularly attractive because they can offer a potential to provide label-free spectral information for detection and identification of the analyte.
On the negative side, miniaturizing microfluidic devices comprised in absorption spectroscopy devices reduces the optical path length and light absorption, and therefore, the sensitivity of the system. In order to mitigate this problem or attempt to achieve the sensitivity of macroscopic systems, some devices have used slow-light photonic crystals or specific waveguide geometries.
One method to address the shortened interaction of light in optofluidic devices on the microscopic scale consists of the use of a microresonator, which is a geometrically shaped waveguide, such as a ring, that allows light to resonate and cycle through the waveguide, before being coupled out of the device to some sort of detector. Resonating the light around the microresonator extends the optical path length of the light interacting with the analyte, and therefore the sensitivity of the device.
Current spectroscopy devices using microresonators have not reached great efficiency or realized the full advantage of their small scale. Some devices defeat the advantage of being small by using some large scale components or requiring other components that could be eliminated. In at least one example of such a device, the device requires detection of light interacting with an analyte at a particular wavelength, and requires multiple, differently sized resonators to obtain spectral information of the analyte at various wavelengths. This device would be many times the size of a device that did not require multiple resonators to obtain spectral information of an analyte for a range of wavelengths (e.g. an absorption spectrum). The ability to conduct massively parallel measurements is also decreased due to the increased size in obtaining, for example, an absorption spectrum. In another example, a device uses a light source that emits a broadband light whose spectral width extends across many resonant wavelengths of light in the microresonator. This device requires the use of a spectrometer to separate different wavelengths in order to ultimately provide an absorption spectrum for the analyte.
It would be advantageous to use one or more cavity-enhanced microresonators in a microfluidic device for absorption spectroscopy by reducing components and size or avoiding large scale components that decrease portability, increase size, and/or decrease the ability to conduct massively parallel measurements.